1. Field of the Invention
The present invention relates to a method for reducing the emission of nitrous oxide during or after a regeneration of nitrogen oxide storage catalytic converters in the exhaust gas tract of a gasoline engine which is operated predominantly lean, a first part of the exhaust gas of the gasoline engine being supplied to a first three-way catalytic converter in a first exhaust bank and to a first nitrogen oxide storage catalytic converter connected downstream, a second part of the exhaust gas of the gasoline engine being supplied to a second three-way catalytic converter in a second exhaust bank and to a second nitrogen oxide storage catalytic converter connected downstream, and the exhaust gas being subsequently merged in a shared exhaust gas tract and being supplied to a catalytic converter for selective catalytic reduction (SCR) and to a third nitrogen oxide storage catalytic converter connected downstream. The present invention further relates to a corresponding device for carrying out the method according to the present invention.
2. Description of the Related Art
For lean operated gasoline engines it is known to provide an exhaust aftertreatment using a three-way catalytic converter and a downstream nitrogen oxide storage catalytic converter. Exhaust emission control takes place in the stochiometric operation of the gasoline engine with the aid of the three-way catalytic converter. The nitrogen oxide storage catalytic converter, also referred to as a nitrogen oxide storage/reduction catalytic converter or NSC, stores the nitrogen oxides which arise during lean operation. Nitrogen oxide storage catalytic converters work discontinuously in a mode which includes two phases: In the first, longer phase, the so-called lean phase (lambda>1), the nitrogen oxides of the engine which are contained in the exhaust gas are stored. In the second, shorter phase, the so-called rich phase (lambda<1), the stored nitrogen oxides are regenerated with the aid of rich exhaust gas which is generated within the engine. During the regeneration, only nitrogen (N2), water (H2O) and carbon dioxide (CO2) are formed from the stored nitrogen oxides in the case of the usual operating mode of an NSC.
During the regeneration phase (lambda<1) of the nitrogen oxide storage catalytic converter, ammonia (NH3) may be formed there in secondary reactions under certain operating conditions of the engine, such as at a low temperature of the three-way catalytic converter. The ammonia causes nitrous oxide (laughing gas, N2O) to be formed in the nitrogen oxide storage catalytic converter. Nitrous oxide has a very high global warming potential. For this reason, the emission of nitrous oxide is subject to a strict limiting value, for example in the USA.
A known possibility of reducing the emission of nitrous oxide is the additional use of a catalytic converter for selective catalytic reduction (SCR catalytic converter) upstream from the nitrogen oxide storage catalytic converter. The ammonia generated in the three-way catalytic converter is stored in the SCR catalytic converter and converted with the arising NOx or directly with the existing NOx during lean operation.
A method is known from the published German patent application document DE102010014468A1 for reducing harmful exhaust gases of a lean operated internal combustion engine by using an exhaust aftertreatment system including a first NOx storage catalytic converter which is positioned upstream and followed by an N2O reduction catalytic converter, including the steps:    a) conveying a lean exhaust gas through the NOx storage catalytic converter during normal operation;    b) supplying an exhaust gas having λ≦1 to the N2O reduction catalytic converter shortly before or simultaneously with the initiation of step c);    c) conveying an exhaust gas mixture having λ≦1 through the NOx storage catalytic converter until the latter is sufficiently regenerated;    d) discontinuing normal operation.
In the illustrated exemplary embodiments, the exhaust gas is conveyed, in this case, in a single-flow exhaust gas tract through a three-way catalytic converter, an NOx storage catalytic converter as well as a final N2O reduction catalytic converter. The N2O reduction catalytic converter may be designed as a three-way catalytic converter, as an NOx reduction catalytic converter, as an NOx storage catalytic converter, or as an oxidation catalytic converter. According to the present invention, rich exhaust gas is conveyed past the NOx storage catalytic converter to the N2O reduction catalytic converter simultaneously or shortly before the regeneration of the NOx storage catalytic converter. In this way, a sufficient amount of reduction agent at the N2O reduction catalytic converter is available for the purpose of reducing the occurring nitrous oxide during the regeneration of the NOx storage catalytic converter.
In contrast thereto, exhaust systems are known which are constructed in a so-called Y system. The exhaust system is, in this case, implemented in two exhaust gas tracts (banks), which are close to the engine and which are assigned to two cylinder groups of the internal combustion engine, and a downstream, joint exhaust gas tract. A three-way catalytic converter (TWC) and a nitrogen oxide storage catalytic converter may be situated in each of the exhaust gas tracts which are close to the engine, and an SCR catalytic converter for selective catalytic reduction of NOx using ammonia as the reduction agent as well as a downstream nitrogen oxide storage catalytic converter may be situated in the joint exhaust gas tract.
In such a Y system, ammonia is formed at the two three-way catalytic converters during the regeneration of the nitrogen oxide storage catalytic converters using a rich exhaust gas mixture. At the two front nitrogen oxide storage catalytic converters, no or only very little nitrous oxide is formed therefrom since they are relatively hot during operation. The ammonia is stored in the SCR catalytic converter, whereby no or only little nitrous oxide is generated even in a cold, rear nitrogen oxide storage catalytic converter. If, however, another lean operation does not take place in such a system after the regeneration of the nitrogen oxide storage catalytic converter, ammonia remains stored at low SCR temperatures and is then discharged at higher SCR temperatures. This ammonia reacts in the downstream nitrogen oxide storage catalytic converter to become nitrous oxide. It is possible in gasoline engines, in particular, that the regeneration is followed by a regular operating phase in the case of a stochiometric or rich air/fuel mixture, thus resulting in the above-described formation of nitrous oxide.
The published German patent application document DE10393184T5 describes a system for treating exhaust gases which are emitted by a vehicle, including:    a) a multi-cylinder diesel engine including a first exhaust manifold in flow connection with a first plurality of cylinders and a second exhaust manifold in flow connection with a deviating, second plurality of cylinders,    b) a first NOx absorption catalytic converter in a first exhaust branch in flow connection with the first manifold,    c) a second NOx absorption catalytic converter in a second exhaust branch in flow connection with the second manifold,    d) a shared exhaust branch having an inlet in flow connection with the first and the second exhaust branches downstream from the first and the second NOx catalytic converter, an oxidation catalytic converter being situated in the shared exhaust branch through which exhaust gases flow from the first and the second branches after being merged in the shared exhaust branch, and    e) an ECU means which controls the composition of exhaust gases in the first exhaust manifold independently of the composition of the exhaust gases in the second exhaust manifold according to a programmed routine for the purpose of periodically generating rich gases in the one exhaust manifold and lean gases in the other exhaust manifold.
The publication thus provides a system and a method for regenerating nitrogen oxide storage catalytic converters in multi-cylinder diesel engines having a Y exhaust system including two exhaust manifolds, in each of which a nitrogen oxide storage catalytic converter is provided. The exhaust manifolds are followed by a shared exhaust branch including an oxidation catalytic converter. To regenerate the nitrogen oxide storage catalytic converters, in a first step, a first cylinder bank is operated rich and a second cylinder bank is operated lean so that in one exhaust manifold, a rich exhaust gas is present and in the second exhaust manifold, a lean exhaust gas is present. The nitrogen oxide storage catalytic converter which is exposed to the rich exhaust gas is regenerated while NOx from the lean exhaust gas is absorbed by the second nitrogen oxide storage catalytic converter. In a next step, the first cylinder bank is operated lean and the second cylinder bank is operated rich, and the second nitrogen storage catalytic converter is regenerated. The control of the exhaust gas composition takes place in both steps in such a way that the joint exhaust gas flow remains lean so that the oxidation catalytic converter is able to oxidize the excessive reduction agent (HC, CO) and thus prevent a reduction agent slip.
According to the description, the method and the system may also be used for gasoline-operated lean engines.
The method thus allows for the emission of HC and CO to be reduced during the regeneration of nitrogen oxide storage catalytic converters in a Y exhaust system. What is not described is the prevention of the emission of nitrous oxide in a Y exhaust system which is formed at a nitrogen oxide storage catalytic converter from the ammonia which is generated at a three-way catalytic converter, which is connected upstream, during the regeneration phase of nitrogen oxide storage catalytic converters.
It is thus the object of the present invention to provide a method, with the aid of which the emission of nitrous oxide may be reduced in gasoline engines which are predominantly operated lean and include a Y exhaust system.
A further object of the present invention is to provide a corresponding device for carrying out the method.